I am cosidering a XCR micro.  I would buy it as a pistol and then after SBR paperwork
goes thru add their folding stock.   I don't understand how the stock is attached  
Can someone clue me in on how the stocks are added.  Thanks

AFAIK, they have a buttcap screwed on by the same screw you'd use to attach the stock.  I never bought the pistol, but I do have two XCR rifles and one PDW upper.  

The stock is just held on with one specialty allen head screw.  There is a keyed groove in the back of the lower receiver that locates the stock side to side.  The bolt just holds it in place essentially.

HTH,
Sean

He is exactly right.

The back of the XCR has what they call a receiver extension.  In the pistol, it's just a rounded hunk of metal with a sling point (I think).  To put a stock on, you need to have a receiver extension that either has the non-folding stock, or the half-hinge.  I would recommend the half-hinge, unless you're in a commie state.  The other half of the hinge would be either on the factory folding stock, or the M4 stock adapter.  The adapter allows you to put any buffer tube and tube mounted stock on.

Although, with the SBR, you might want to just stick with the shortest LOP factory folder (9" maybe?).

That's exactly what I did with my micro.  If you'll watch the classified on the XCR forum, you can usually find a complete stock for sale or you can buy one from RA or Kermit.  Don't worry too much about the length of pull as it is pretty easy to disassemble the stock and cut it down.

Here's my finished mciro: