Transfer Order BU Security

Hello,

We have multiple regional companies set up.  And, we are using BU security to limit access.  For example, company A users cannot see/create orders in Company B.

However, we now need to open up BU security for SK/OK process to allow creation of OK.  As such, Company A users can now create SO orders for Company B.

Any ideas how to prevent this?  We only want to open up BU security for SK/OK process.

Thanks.