Would a bad modulator valve make a 2000 Lumina transmission slip only after it gets hot?

Modulator valve affects shifts... not slipping... which is a function of accumulator pressure. And that is affected by if thre is any friction material left... and everything that can affect fluid pressure.

So slipping isn't something minor... automatic transmition bands don't have much material on them... and it only takes a little slipping to wear that little bit away.

Have you checked the fluid level? Has it ever been changed?

It's more likely that one or more clutch packs are shot, especially if you have not been changing the fluid and filter regularly.