I have a Zipp 808 20H wheel with a Powertap hub running a 10-speed cassette. I’m looking to get SRAM Force AXS, can I put a XD or XDR free hub on and run the 12 speed cassette? Or must I get a compatible wheel or freehub?
If it is an older Powertap freehub, it will not work as it would be too narrow for the 12 speed cassette. If your current 10 speed cassette fits on your current freehub without any spacers behind it (or a 1.0 mm thick spacer* for Shimano 10-speed road cassettes), then it is too old (too narrow) as is.
However, if there is a 1.85 mm spacer behind your 10 speed cassette, then your freehub will support it. If you have a Shimano 10-speed cassette, this 1.85 mm spacer would be in addition to a 1.0 mm spacer (two spacers).
If your freehub is too narrow, you may contact Powertap (actually SRAM, as SRAM purchased Powertap a few years ago) to see if there are any options to update the freehub to 11/12 speed compatibility.
*Note: the 1.0 mm spacer for a 10-speed Shimano road cassette is used for it to fit a standard 8/9/10 speed freehub. The 10-speed cassette is actually 1.0 mm narrower than 8- and 9-speed cassettes.
A very informative table of cassette-freehub compatibility can be found at this link (scroll down to it) Cassette How-To - Part 2 - Slowtwitch.com