How to Add Crop Marks and Export a 9"x12" PDF in InDesign

Thinking about why yuou might want 9 x 12 I'm guessing maybe you are printing 2-up on 12 x 18 stock.

Pretty much any size should work as long as it gets centered on ecah half of the print sheet.

do you really mean 9 x 12? That's not adding the same amount in both directions and the only way I can think to do that is to add slug  or bleeds that are uneven as @James Gifford—NitroPress suggested.

If you want to add an equal amount around all edges, the easy way is to change the offset of the marks (which by default are too close anyway).  My printer asks for an offset of 0.2917 inch which reults in a half inch addition on each edge, so your 8.5 x 11 would wind up att 9.5 x 12.

If you go that route, define a new export preset (File > Adobe PDF Presets > Define...) withthe new offset so you don't have to remember waht the value is.

Adding crop marks is usually a function of export to PDF, which has extensive control over size, placement, spacing etc. (see the Marks & Bleeds panel of the PDF export menu).

Net size of the document is usually irrelevant as printers (digital and the ink-stained kind) will automatically center it on a print sheet, or impose it on a master sheet as needed for cutdown. If for some reason you need the doc to be exactly 9x12, you can set bleed width to pad it out (1/4 inch on each side, 1/2 inch top and bottom). But there should be a good reason to even bother, given how printing works.