Assuming X and Y never exceed three digits, this should work in a column formula:
Repeat( "0", 3 - Length( Char( :X ) ) ) || Char( :X ) ||
Repeat( "0", 3 - Length( Char( :Y ) ) ) || Char( :Y )
Edit:
Or maybe this is more efficient:
Right( Char( 1e6 + :X * 1e3 + :Y ), 6 )