How do I reinstall round 6" duct pipe in my basement?
Fix

If the pipes have been deformed (by placing something heavy on them, for example) then you can try bending them back to a round shape. If they're deformed to the point they actually have folds or creases in them, I'd give up and throw them out.

New ducts

Round ducts are sold as sheets with a special fold along the two sides that interlock into each other to form the round shape. They're actually very easy to assemble, and not very expensive. Most big hardware stores will have a few sizes in stock, in addition to elbows, boots and other parts.


Source: http://www.familyhandyman.com/heating-cooling/round-ductwork-installation-tips/view-all

You mentioned you don't have enough now anyway, so if you buy a few new pieces, you might be able to use their round shape to fit your existing pieces into. 

Re-crimp ends

You can also get a duct crimping tool to redo the crimps on the end, making them tighter so they fit into the next piece over. 



Personally, I've done several modifications/changes to ducts like this as part of basement, garage, and laundry room renovations, and though I'm generally always happy to have a an excuse to buy a new tool, I've never bought a pair of these. 

A handful of times I've had to make a crimp more tapered to fit pipes from different eras together, and a couple times I've had to completely make my own crimp, but I've just used a pair of long needle nose pliers -- grab on, twist 45 degrees clockwise, move the pliers over 1/2", repeat until you're back at the start. It doesn't look as nice but it's totally functional. Just be careful of having too much protrusion into the duct as it can disrupt airflow. 

Ideally all your crimps are also facing the direction of the airflow, but this isn't always possible. 

Connections

You can use 1/2" self-tapping screws to attach ductwork together, usually a couple is all it takes. For dryer vents, it's better to avoid screws as lint will catch on them and block the pipe. 

Don't forget to tape the seams using aluminum foil tape (not regular duct tape, which is not for heating ducts!). If there's no chance of the ducts being mechanically disturbed (eg, they're in a joist bay, secured by hangers) I've often only used duct tape (no screws) to attach them, but I don't know if this is kosher.