how to pass string variable as globbing argument to command

If you want globbing to be performed upon parameter expansion (which by the way is performed by the shell, prior to calling the command; the command sees the resulting files, not the pattern), that's where you'd leave the parameter expansion unquoted.

Leaving a parameter expansion unquoted implies both word splitting and globbing (sometimes referred to as a conceptual split+glob operator). Here, you only want the globbing part, so you'd need to disable the splitting part (by assigning an empty value to the special $IFS variable):

IFS= # disable splitting
for i in ./*\ P1.tif; do
    match=${i/\ P1/\ P*}
    out=${i/\ P1/}
    echo "processing $match"
    tiffcp $match "$out" # split+glob on $match
done

That assumes the leading part of the file name doesn't contain wildcard characters. Alternatively, you could do:

for i in ./*\ P1.tif; do
    prefix=${i%\ P1.tif}
    out=$prefix.tif
    echo "processing $prefix"
    tiffcp "$prefix P"*.tif "$out"
done

That way, only that * is taken as a glob operator, and that code also has the advantage of only using standard sh syntax.

Note that glob expansions are sorted lexically, so P10 would come before P2. Use zsh and its numericglobsort option (or n glob qualifier) for numeric sorting.