Can you program an RCX using the Mindstorms NXT 2.0 software?
No, the standard NXT-G software provided by LEGO will not allow you to program a RCX brick.

Outside of official software, your best bet is probably to use NQC which is C-like; there's also NXC for the NXT which is similar.

There are a lot of others which have been discussed in the following questions: Is there any other way to program the Lego Mindstorms NXT other than the provided software? and Can I write programs for the LEGO RCX 2.0 in Java/C++?.