The Legend R7 manual says:
Interdigit timers are used by the MERLIN LEGEND System to determine when a user originating an outside call has completed dialing the digits. The information is
necessary to allow the system to perform subsequent operations.
which makes it sound more like the Legend is listening to the user's dialing, rather than controlling the duration of the tones.
Which DTMF tones are you trying to extend? Those that the user dials directly, or those generated by something like a speed-dial? What problem are you trying to solve? A CO circuit that isn't reliably receiving DTMF?