Garmin Gps Battery Modification
What you are suggesting is fraught with difficulty and danger.  There will be an intelligent battery charger/monitor either built into the battery pack or on the Garmin baseboard.  This will have the characteristics of the supplied battery built into it, so replacing the battery with a larger capacity one will make the charger/monitor operate incorrectly or give an incorrect battery status.  Without knowing exactly how the battery pack information is used by the Garmin I cannot tell what the effects will be.

Many equipment manufacturers have taken to adding an encrypted battery ID into the pack to prevent the use of cheap, poor quality replacement battery packs.  If this has been done then your Garmin will reject the replacement battery.

A solution to your problem could be to use a battery pack like this one from Duracell, or for you to design your own solution to feed 5V in through the USB connector.