I think we MUST be careful what is added to a name.
Examples - in my family, several men had the given name of "Doctor" or "Dock." A copier/clickologist gave them all an MD suffix. I have a cousin whose given name IS Captain.
As you might guess from my username, one of my direct lines is the Burgess family. In an early twig, one of my ancestors was an elected Burgess (title, not name). Many trees list him as Burgess Larkin Chew (given name, middle name, surname), while his name IS Larkin Chew (given name, surname). Do you know what that does to hints for my Burgess line?
The OP suggested (Captain) or "Captain" - the first would make it appear that Captain was the maiden name; the second, a nickname.
I could possibly see using the suffix box for a title that does not change over time - for example, a minister would still be a Reverend. But military ranks change over time, as do most other occupational titles.
In the south, after the Civil War, many men of status were called "Colonel," while having only served as privates in grey.