Not for the 7510.

The only Blackberries that an external keyboard is currently pratical is the 85X, 95X and the 62XX series, and the keyboards exist.

However, the other Blackberries (especially 7XXX series), do not support connectivity is necessary for an external keyboard (at least easily).

The next Blackberry that this may be pratical on, is the 7290 with Bluetooth, assuming support for designing a Bluetooth keyboard driver.