First note that migration paths being restricted to close voters by default is by design.
There are a lot of pitfalls when migrating¹ and it’s more efficient to train a few moderators on those² than all close voters.
Therefore migration paths are only created in the rare case that the amount of migrations between two sites exceeds what moderators can handle. To cut a long story short, the following is not a work-around but the intended way:
To effect the migration of a question, do both:
Flag or vote to close it with the not within the scope or a custom close reason elaborating why it is outside our scope.
If the situation is not crystal-clear, the latter is strongly preferable.
The argument for this should be independent of the existence or scope of other sites – “it is on-topic on [Topic] SE” is not a valid close reason.
Also, if the question is closed for any other reason, migration will not solve its problems.
Raise a custom moderator flag, stating where and why the question should be migrated.
¹ You got to have know a minimum about the scope of the target site, keep in mind the golden rule of migration (mind that I am not saying that this applies in your specific case), etc. Half of the migration flags we get blatantly ignore these. Also, for many migrations feedback from moderators or members of the target site is sought before anything happens.
² This is not only because mods are fewer people. They also have special tools to ask other mods and, in turn, they can be easily contacted if they make a mistake migrating and learn from it.