I'm new, excuse me if this has been addressed, couldn't find it in the first 3 pages.
At the cathedral in my diocese, we often do both an Entrance Antiphon and a Hymn, usually for celebrations with long processionals. Sing to the Lord doesn't forbid it, nor does it make it clear that one shouldn't do such a thing, yet my pastor (who is also a bishop) says it's wrong.
I think that as long as it doesn't slow down the Mass - and usually in longer processions, we still need the organist to cover the procession, especially if incense is involved - there isn't anything that says we can't do it that way.
By the way, welcome to the forum. And there's a handy search box at the top right of the main forum page that can be used to search for topics using keywords. That's how I found the thread I linked to above. Hope you enjoy participating here. It's a great place to learn with lots of knowledgeable and generous contributors.
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