US Company with Canadian Subsidiary

US Holding company owns several subsidiaries and just purchased a company in Canada.  The employees of the Canadian subsidiary are still paid via payroll through the Canadian subsidiary, but the company is funded via the US holding company.  Can the US holding company include the employees of the Canadian subsidiary in their 401(k) plan?  My gut says no because their income wouldn't appear on the surface to be US sourced, but I'm on the fence.  Anyone run into this before?  Thanks in advance!

I don't know all the particulars but do know that participation of Canadian resident citizens in US retirement plans have a lot of complexities that can create both tax issues and Canadian retirement plan issues, including their version of social security, for those individuals. We had to grapple with that on pension plans. It is much easier not to include them.