I have a building that has 20+ outdated panels in different shops. I can get new guts for the panels. Question: If the feeder wires are not long enough can I splice onto them or do I need to pull new feeders? Pulling new ones may be difficult bcause they are underground in a 30 year old building.

Yes you can splice a feeder.

Yep, an easy and clean way to do it is to put a large pull can the width of the panel above or below the panel with one or two over-size nipples, and take care of all your feeders and other 'too short' wires there too. Make it really clean with some terminal strips for the branches that can't make it either, and insulated lugs for the feeder.