IRS rules are very complicated. you really need to talk with professionals that specialize in real estate Ira's.
The short answer is yes. You can merge all retirement accounts to maximize the cash amount available for purchase. If your IRA now has lots of cash, or stock that can be sold for cash, you can pay cash for the real estate. That, of course, gets you around the UBIT problem because Leveraging or mortgaging to buy a property is a taxable event in an IRA.
This is a short quote from an article on the your property path website in the financial planning for real estate owners section.
I'm pretty sure you can only purchase investment properties and keep the money part of an IRA. I believe there are some pretty strict rules related to it.
An IRA can't contain your residence, so if you took money out of an IRA to purchase a residence it would not be an IRA any more, and you'd have to pay the taxes and possibly a 10%penalty.