Is there a way to find this out before I accept the job offer? I'd seriously reconsider if I knew for sure that they wouldn't allow it, but I don't want to risk the whole offer by asking them outright.

If something is important to you, you simply need to ask about it.

Something like "My partner and I have a dream of living overseas for a few years. Is transferring to X something that would be allowed here somewhere down the road?" should get your answer. If the answer is a positive one, you may or may not want to follow up with something about "within a year" if your time frame is set.

It's okay to jeopardize a potential offer for important aspects of the job. Offers that don't offer the possibility for something that you consider important enough are ones you should accept anyway.

Is there a way to find this out before I accept the job offer? I'd seriously reconsider if I knew for sure that they wouldn't allow it, but I don't want to risk the whole offer by asking them outright.

If something is important to you, you simply need to ask about it.

Something like "My partner and I have a dream of living overseas for a few years. Is transferring to X something that would be allowed here somewhere down the road?" should get your answer. If the answer is a positive one, you may or may not want to follow up with something about "within a year" if your time frame is set.

It's okay to jeopardize a potential offer for important aspects of the job. Offers that don't contain the possibility for something that you consider important enough are not ones you should accept anyway.