How do I add existing domain to an existing forest without impacting users?

Mike, you type better and read better than I do :)

I was mis reading that sanukc was trying to create Everett domain, or it's already been created..

All 3 locations are different companies.  My team manages the domains for all 3 locations and they need to be kept separate.  That's the reason for the 3 domains.

hummm..if all three locations are different companies? Do they have have anythig to do with each other? Like are theire business merged or plan to merge? If not, you should keep them separated. I don't think you should even create trust between them. Unless if they are partner or has something to do with each and agreed to share resources between these three businesses then you can create trust between them. Just creating trust will not impact any forest domain and allow them to share resources.
Otherwise, for security reason an these businesses has nothing to do with each other, there is no reason to create trust, you need to keep them separate for sure.

I agree with Americom, keep them separate until you have a requirement to create a trust or merge them.   For instance if users need to share resources then you create a trust.  As long as none of that is needed just keep them separated.

The only reason I was thinking of merging (or at least creating a trust relationship) was that our CFO (of the whole company) travels to all 3 locations and would like to use resources at all 3 locations like printers and fileshares.  This is not a drop-dead requirement, but he has asked for resources many times and one of these days might lose patience with me for not allowing him to print on the printers wherever he is.

Well then that would be a requirement, or if more people start going to different locations.
How do you work it for him now?  Does he have three accounts?

may be we are no too clear on the relationship of your buiness in related to the three businesses. What does your CFO has to do with the resources of those three companies, particular with fileshars? Does your company also have another separate domain or use the domain user accounts in one or more ot those two domans?

The 3 companies have controllers who report into a CFO of the parent company.  The CFO travels between these companies on a weekly basis.

I allow him to log into an Everett account when he visits the Everett office.  He copies files he wants to print to an USB key and logs into an Everett system to print them.

Since the Victoria domain is trusted and part of the CompanyForest, he is able to use resources in Victoria without issues.  

It sounds like these companies have operations need for certain invidual such as the CFO. May be later, there will be more request similar to this. Then like Mike said, there you have a requirement. I guess for now, the best way is to just established trust between these Domain Forests and observe how heavy and frequent the request over time. I'm sure in the future, if there's a need, you might end up migrating these domain into one but it seems like that would be too early to call at this point. So, just creating trust between these domains would make your CFO happy. This way, he can only need one account and have access to all resouces in all domains if needed.

At least one good thing here is if you do need to do a major migration down the road the CFO does control the money and funds...thining about money  for 3rd party migration tools :)