lreplace

lappend <list> <start index> <end index> <new list or element> <new list or element> ...

This function deletes the elements from <start index> to <end index> (inclusive) and replaces them with the new specified lists or elements. All elements must be Efinity Tcl objects.

set lut_list [get_cells LUT*]
set ff_list [get_cells *~FF]

# Replace the second to fourth elements of $lut_list with elements in $ff_list
set combined [lreplace $lut_list 2 4 $ff_list]

# Append the list $ff_list to the end of $lut_list
set combined [lreplace $lut_list end+1 end+1 $ff_list