If I understand correctly, you are wishing to insert a pause between # and 700.
Unfortunatelly, no goot news for you. It is possible on SPA[35]xx phones starting with 7.5.1 firmware, but I suspect the SPA9xx is so old to support it.
As a very dirty hack, you can change string to something like
sub=#0000000000000000000000700
then modify Asterisk's extension conf to ignore all but last three digits.