DECLARE @STR varchar(100) = 'FN,SP,TBL,091.889'

DECLARE @Separator VARCHAR(10) = ','

DECLARE @ValueAdd VARCHAR(10) = '80990'

DECLARE @Action VARCHAR(10) = 'R'--1. P..PostFix 2. R..Prefix

SELECT @STR =

STUFF((

SELECT

CASE WHEN @Action = 'R' THEN ISNULL(@Separator+@ValueAdd,'') ELSE '' END

+

ISNULL(@Separator+@Str,'')

+

CASE WHEN @Action = 'P' THEN ISNULL(@Separator+@ValueAdd,'') ELSE '' END

),1,1,'')

select @STR