Adding Canada Employees to PBGC covered pension plan

Does any one work on pension plans that cover both US locations(US Citizens only) and Canada locations (with Canadian citizens/employees only)? 

I know it is allowed and possible, and that there would typically need to be provisions that may apply to ONLY the Canada employees, and ONLY US employees. 
There is an employer that sponsors a traditional DB plan, it is PBGC covered, and they are interested in expending the pension benefits to include their Canadian employees. 

Does anyone have some technical reading they can recommend? or if you do this kind of work do you have suggestions? TPAs I can suggest they look at that do this kind of niche work? 

Other thoughts? 

Bridging vesting service across Canadian employment and U.S. facilitates transfers across border

Isn't there a requirement that they need to have US based income?

1 hour ago, Jakyasar said:

Isn't there a requirement that they need to have US based income?

there isn't a requirement that I'm aware of. 
Here is a discussion that touched on that a bit. 

I understand the non citizenship/non resident exclusion but again, not sure about the lack of US based income. May be an exemption for Canada??

A foreign employee working with an H-1/H-2 visa and have US income is included for testing purposes. May be excluded from the plan categorically.

Also a foreign employee working outside US but the income is from US is also included for testing purposes.

The above is based on a multi national client that I worked with and had a good ERISA attorney involved with all this but this was 10+ years ago so commenting from memory.

Things may have changed or I may be remembering wrong but one thing always stuck with me was US based income.

If the plan does not exclude them, my understanding is that their compensation is converted to a US dollar equivalent for purposes of the plan.  

John, are you saying if they made 140k CAD of salary in Canada (not a US based income) and the equivalent of it which is roughly 100k USD, the US company can make a contribution on their behalf for a pension plan maintained in US under IRS/DoL regulations? If that is what you are saying, where would they deposit the monies (apart from what currency)? Would the US company even be able to take a deduction for it? I must be missing or not understanding something here.