Keith,
I haven't done this task. The tube headers are pretty thin, but what you want to do is 'do-able'. There are things called 'weld flanges'. You can get them through McMaster-Carr in the pipe size of your choice...probably 1/4" or 3/8".
If you can't find weld flanges, you can use steel couplings instead, they just don't look as professional. Make sure whoever welds them in knows what he/she is doing...the wall on tube headers is pretty skinny, and even skinnier on used ones.
Stuart
