declare	@string		varchar(20) = 'abcdefghijklmnopqrstuvwxyz',
	@sepratorpos	varchar(10) = '4,6,15'

declare	@newstr	varchar(100)
select	@newstr = isnull(@newstr, '') + ch
from
(
	select	v.number, ch = substring(@string, v.number, 1)
	from	master.dbo.spt_values v
	where	v.type	= 'P'
	and	v.number between 1 and len(@string)
	
	union all
	
	select	numberval, '-'
	from	dbo.CSVTable(@sepratorpos)
) a
order by number, ch

select	@newstr