Electrical circuits mixed in same conduit
It's a little tricky. You will get mixed interpretation about NM cable (including Romex brand) in a raceway, if you determine you can then you calculate the resulting percentage of conduit fill based of the size of the NM cable assembly as a single conductor, but bottom line just don't do it (unless you raceway is just a short protection sleeve), it's much easier to pull #10 stranded THWN wire than a large solid wire NM cable through a conduit that already has wire in it.

Then with 4-6 current carrying conductors get de-rate the ampacity of the wire to 80%, so your existing wire may become under-rated.

NEC 312.8(A) Restricts pulling conductors through a panelboard cabinet to a maximum 40% cross section fill for conductors, and 75% fill for conductors and splices. A label must be attached to indicate closest disconnecting means for through conductors.