Do you ever/is there any way to: Expand your ballpark to seat more people?
 

And if so, how, and why?

Go to League Setup and Edit Ballparks. Find you ballpark and change the capacity to whatever you like.

Note: I don't if you have to be in commish mode to do this, but as I only play in commish I wouldn't know any better.

Bigger question - if I play GM only, is this cheating?

Pay for it.

Not in my opinion. Since all of my parks are real in my EBL league I use the .prk files by Pstricket and just import new factors every 3 seasons. So for my current team, which plays in a version of Old Comiskey, i used park factors olf Comiskey from 1973 in 2003, 1976 in 2006 and so on. Capacity has increased by about 600? I may be slightly off. Fences came in that much I know.
I change mine more to keep up with my strategy which started in 1973 for 2003. at 1985 for 2015. Only 2 parks that I use in 2015 did not exist in 1985. Polo grounds and LA Coliseum which I have not changed at all. Seats 77,000 They have ranked #1 in attendance every year. Largest city also the best team.

I added retractable roofs to all the parks. I do not like rain outs.

I think you mean rain delays. Games cannot be postponed in OOTP.