StringInsert

StringInsert["string","snew",n] yields a string with "snew" inserted starting at position n in "string". 

StringInsert["string","snew",-n] inserts at position n from the end of "string". 

StringInsert["string","snew",{n1,n2,…}] inserts a copy of "snew" at each of the positions ni. 

StringInsert[{s1,s2,…},"snew",n] gives the list of results for each of the si.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.