Serious answer first...
There's no problem mixing/matching gear, as others have said. I do it all the time- it's usually cheaper for me to cop the import tax than to buy locally...
I'd also check to see if you really need a step-down transformer. The majority of gear these days can accept from 90-240 V without a problem, or ot least let you switch between 110 and 220 V (you can usually get away with 220 gear in a 240 socket).
The only bits of Kit I find require step-downs are ones with transformer-style PSUs, or highly-specialised PSUs, like you'd find on high-end Pres etc...
The analouge audio voltages are fairly standard, 1 dBu = 1.77V (or something similar, I'm going off the top of my head here) and that figure is used to derive +4/-10 dBu line levels, and -20dBu mic level.
In other words, connecting two bits of kit with an audio cable will be fine, even if they are from different countries (considering the majority of audio gear in most people's scope is manufactured in asia.. whatever...)
The only probelm I can see coming from this is that if you've got power coming from two sources you are setting yourself up for a ground loop. Easy way to rectify this is to make sure your step-down is running off the same supply as your other gear.
Now, allow me my moment of levity and yank-bashing...
Your ameircan gear will up and invade the hell out of any gear you buy from anywhere else.
Also, they will call the French cowards even though it wsa the french that fought and won the war of independance for them...