insert two columns in string

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i have a string of columns containing different data

i wish to insert and use a column after a certain string as an additional insert and rewrite the string

example--this is a comma delimteed string

input string

search string  '3699' and the subsequent data following '3699' is to be used when i insert just before string '3699'
string '3698',followed by the data that 3699 has

example of data
2010,abcdef,3699,20000,4010,6776

the updated string should read
2010,abcdef,3698,20000,3699,20000,4010,6776

i could send you the real data
many thanks
david ???