I have a Milwaukee Portable Bandsaw and have kept the old bimetal blades as I replace them.  Is it possible to use these for pattern weld (using a contrasting steel, like strapping) or is this project doomed?

Try the heat/quench/break test and see if the backing metal is high enough C

If it breaks that's a good thing, right?

Bimetal means the blades are mild steel with a higher carbon edge.  These would NOT make good blade material as they have mostly mild steel content!  They are made that way to save on the cost of steel and also for flexibility/break resistance.  Since you already know that they are bimetal blades, testing them seems a bit superfluous.

I've seen a number of bimetal blades where the backing steel was definitely not mild---which is why I suggested testing them.

i thought that the bimetal blades were carbon steel back and high speed teeth? 

What Dave said in most cases.

I have tested several different brands of sawzall blades all of which are bimetal and all were steel throughout. I am currently unable to weld myself do to my forge dying on me but I have welded them up before and it makes for a tough knife.

Jcornell suggestion would be to keep it hot while you work it. If it gets too cool while hammering you'll probably mess it up. I found a few failed welds because of that.

One problem you may run into, some blades will not weld together.  You 'll need to alternate.  I've done it by cutting the blades into 7" segments then taking some pallet banding and basically folding it together.  If you do it right you wont even need to wire or tack weld it together.  When I first started doing the process I had trouble on the third fold.  I finally figured that the mild steel had oxidized off and the band saw was exposed, it wouldn't weld too itself.  I then figured that taking a small file and placing it in the center would do two things, it would juice the billet and allow that final weld to take.  works well.  The only problem I had after that was the pattern was rather muted.

^ what JMC said.

Fwiw, I think you should just scrap unknown material and go with a known...unless you don't care about your time/fuel etc. I'm a great believer of using found materials if I have an inkling as to what I'm working with, ie, leaf springs etc. That being said, I'm finding that as I get older, I really don't want to waste my time for some nebulous result.

John