I woudl imagine so, although I expect you'll have to down your ethernet card before dial'ing up on the external modem - otherwise you'll get confusing 'default gateway' / broadcast / rooting loop problems.
But theoretically you should be able to do thusly :
surf on adsl
ifdown eth0
pppd on
putz around on slow modem
pppd off
ifup eth0
Slick.
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