I wanted to get rgb led strips in my case but the ones that run off of molex are expensive so I want to use a strip powered by a 12v jack and solder the cable to a molex, would I be able to do that?
Should work, no problems
Make sure the strips are rated for 12v though.
Soldering directly onto the molex pins would be kinda tricky, as the solder sometimes won't stick properly. I suggest you pluck out an old molex connector of an opposite type from some old computer junk, strip the wires about an inches length from the molex part, and then solder the led wires to that. This way you can easily disconnect and remove the LEDs when not required