Scott,
I have checked the XD3 manual and it looks like the only possibility to import new sounds is through these flash cards. At first I thought that they worked like expansion of the internal ROM, but then at the end of the manual (page 97), under "Specifications / Optionals", I have found a reference to "Flash sound expansion (8 or 4 Mb) blank or preloaded".
Maybe it's too early to ask for a clarification on this issue; I think that for now it's safer to assume that if importing new sounds is a priority then the SD1 could be a better choice.