From memory, you'd have to complete the OWSI portion of the PADI Instructor Development Course, plus a few additional academic modules because you are a crossover candidate, and then sit the PADI IE.
For a seven day IDC, the OWSI portion normally lasts four days. So from a purely financial perspective, the cost of crossing over is not that different from just becoming a PADI instructor in the first place (you have to buy all the manuals and other materials).
I personally think that "becoming an instructor with one agency, whilst planning to cross over to another in the future" is a bit nuts - again from a financial perspective. You're probably better off just choosing the agency you want to end up teaching for, and go that route.
There is always the chance that you will cross over to another agency "at some uncertain point in the future".... but I'd worry about that when it happens, rather than now. Who knows how the agency standards will change in the future.
If becoming an SSI instructor suits your current situation (and immediate future plans) then go for it.
If you have a date when you want to be teaching PADI by, ignore the SSI step and just go for the whole PADI IDC when you are ready.
Hope this helps.