you can get adaptors or build one, is the reason that you want to as you dont want to throw them away as you have a boot build and you dont want speakers sharing enlosures (sub and 6*9 interaction). If so, my advise would be, see the 6*9s and buy a pair of comps, a decnet pair if you can spend about 150, if you have an amp, amp them up, this will be better as most 6*9 are not very good quality as they are made for mass market, and are used often by people who put 3 sets in the parcel shelf. There ARE good ones out there if you have a good pair i would still recomend selling them as it will be much easier to put some comps in.
But yer its poss, i have a pic of it somewere, if i can find it il post it
sorry about the rambling!!
Rorrie