The only way to do that would be to have a custom API script. If you only ever roll one extra die on a 6, you could do it with a rollable table, having numbers 2-5 each having a weight of 6, and 7 (6+1), 8 (6+2), 9 (6+3), etc each having a weight of 1. You'd then have a value of -3 instead of 1, with a weight of 6. If you can roll up more than one time, you can still do it, working out the needed weights, but it gets clunky, and you have to deice what the maximum number of dice you'll calculate the table for. You can use rollable tables like dice in macros, with 1t[table-name] for 1 roll, and /roll 1t[table-name] +3 will do a pseudo-dice roll.