"Making a total of" is acceptable, but a little verbose.
Your other option uses a similar looking verb that has an incorrect meaning. Totalize means⁰ "to comprehend in a way that includes everything" but you intend to convey "amounts to a sum of" which would be "totaling." So your sentence would best be written:
The study included 30 hospitals and 3 clinics. For ease of communication, the 3 clinics will henceforth be regarded as hospitals, totaling 33 hospitals.
⁰Commenters have suggested that, in some places, totalize can indeed mean "amounts to a sum of." This may be true but using it in this way risks confusing readers who understand its meaning only as I have indicated. So, even if it's technically acceptable for some, it is best not to use it in this way.