Running 6/3 Romex into 1" LB Conduit
Reviewing some of your added info...

Since you only need 50A, you need #6 in Romex (NM-B or UF-B) cable... but in conduit you can run THHN or XHHW wires at a higher temperature, so you can get away with 50A on #8 copper or #6 aluminum.  At 50A, Your ground only needs be #10 Cu, or insulated #8 Al. 

You could also stay with the 70A thing if your hot tub subpanel provides 50A protection and it has extra spaces. That would allow you to add another couple of circuits later. 

You can transition between wiring methods anywhere you can put a sufficiently large junction box that will remain accessible.  What happens if your NM-B cable only sticks its neck out 1 foot into outdoor space, just enough to get into an outdoor box?  That's a matter of some disagreement here, but the understanding I have is that is not allowed. Water will wick for dozens of feet down the paper wadding, and will rot the cable from the inside. 

PSA: Take no shortcuts when wiring a hot tub: while electrical drownings are deadly, bad hot tub wiring can also kill people 3 towns over.  Anywhere in the wildfire west, I would use metal conduit to wire a hot tub, and I'd run ground wires also. 





Conduit is not actually liquid-tight.  In fact, you are required to assume outdoor conduit is 100% full of water 100% of the time!  The protection must be in the wires themselves.  

You would need 2" conduit for that

By "Romex" what you actually mean is UF-B cable, which is rated for outdoors/underground/in outdoor conduit.  UF cable is wide and flat, and as such requires quite large conduit -- 2" in fact.  At that point you might as well run fatter aluminum MH feeder for half the cost at the same ampacity.  

If you really wanted to get inside 1" schedule 80

... run Aluminum XHHW 4 AWG at 25 cents a foot per wire (x3) plus a 6 AWG AL ground wire (20 cents a foot).  At that point you can breaker for 70A.  

This involves a couple of reality shocks for many people: first that aluminum wire is actually perfectly OK for feeder.  And second that individual THHN/XHHW wires are not only a legitimate wiring method, but very much the preferred one inside conduit! Much easier to pull - and pulling difficulty matters a lot when you DIY.  

Now you'd make that splice inside the house before the wire enters the conduit. The Polaris splices would be about $10/pop x 3, but the savings in wire will pay for it.  Then from your main panel to this splice box, run #6 Cu if you want 60A or #4 Cu if you want 70A.  Or you're welcome to carry the 1" conduit all the way back to the main panel, in which case no splicing will be needed and you run the Al straight from the breaker to the subpanel's main breaker.

It should be noted that #4 and larger wire does not require "the color game". You can simply buy 3x your required length in black, and use colored tape to mark your phases, neutral and ground. However your ground needs to be either #6 Al or #8 Cu (Cu can be bare, Al cannot), because 1" conduit won't quite fit four #4. 

Other notes

Of course an outbuilding needs a "main breaker" in the panel, simply to function as a disconnect switch - nobody cares what number is on the breaker.   It is highly appropriate and correct to use a 100A++ subpanel on a 60A feed. Some people believe that's not true and that panel rating must match feed breaker. No, if you get 112 mph rated tires, that doesn't mean you have to drive 112 mph!

Make sure to get a LARGE panel.  Spaces are dirt cheap when you buy the subpanel, but running out of spaces creates an expensive problem later.  The worst case is when people unnecessarily spend a fortune on copper wire, find themselves unable to pull the balky cable through the conduit and have to hire an electrician to finish the job, and then chintz out on the subpanel and wind up completely out of spaces and having to do expensive rework (or more likely simply be unable to do their next project).

I'm not against direct burial cable.  Much the opposite, I'm a huge proponent of it unlike most here, and the reason is theft.  But cable in conduit is a bad idea unless you wildly oversize the conduit.  Why do that when you can simply learn THHN/XHHW?