Wendy, I'm not sure you are getting the correct advice. You have said you want to vertically concatenate. outerjoin does not do that. It's a completely different operation, closer to horizontal concatenation.
If you do want to vertically concatenate, you do that the same way with tables as with anything else in MATLAB: [t1; t2]. No conversion needed. However, the variable names in the two tables do need to be the same. You have said, "They both have exactly the same column headings", so that should not be a problem. You also said, "the headings may be numbers, categoricals or text.", which doesn't make sense, so I think maybe you meant to say, "the variables in the table may be ...".
So, if you want to vertically concatenate, you shouldn't use outerjoin, and you don't need to convert back and forth with numeric.