It seems you’ve re-written your original question. Please don’t do that! It makes it difficult for anyone who tries to follow the thread — besides making it very confusing for those trying to help you. See the recent posting On node obliteration.
In a previous post you say the fields are space separated, now they’re not? Then how do you distinguish one field from the next? Also, you say you want to insert spaces “from pos[i]tion 15 in all records”, but in the data originally shown, none of the fields was more than 13 characters long. You will need to provide sample data showing the way fields are set out on each line. (And please remember to enclose the data in <code> </code> tags.)
As for inserting spaces (or any other string) into an existing string, you can use substr, as shown by flexvaultbelow.