Changing the Text of the Eighteen Blessings

Is it possible to change the text of the prayer, to add, (and to omit)? I shall note that I do not intend to make changes in the first three and last three blessings, nor the openings and closings of blessings, but rather in the middle of the blessing. And if this is not allowed, how does one innovate in his prayer? I would also appreciate sources, thank you in advance.

Dear …!
It is forbidden to change the text of the blessing that the sages have established, even though, after the fact, if one recited the essential meaning of the blessing, even though he changed the exact wording, he has fulfilled his obligation. However, the early commentators and the Tur have already written that the text of the blessings was written with great precision, with an exact count of the words etc., and therefore, ab initio, one should not change the text of the blessings at all. The meaning of the words “to innovate in one’s prayer” is to add requests according to the subject of the blessing, which is permitted.

See the “Biur Halachah”, section Orach Chaim, Ch. 107, subparagraph 2.