You will get all kinds of advice against this on the Forum, because most of the people here don't know how to weld and think metalurgy is a mysterious black art.
If your dad's
real good with the TIG torch or wire feed (bike tubes are verrrry thin), he may be able to do just that. You may be wise to fire mom's oven up and re-heat treat it when you are done, but - yes - it is possible. Hey, the frame's a gonner otherwise; why not experiment?
(Now watch me get flamed big-time for giving you that advice
)
EDIT - - - DO check further into the BottleRocket. Transition is an awesome company, based in Ferndale, WA (near the BC border) Owners Kevin & Kyle are absolutely fantastic guys - - and very good riders themselves. Their commitment to quality and value-for-the-dollar is second to none.