It's already possible, in a way; The limitation is in the settings of the numericupdown control.
If you enter a value using the keyboard, then click "OK" without touching anything else, you can enter more digits:
Set decimal [dec] value to 0,00000000002
Set decimal [dec] to [dec] plus 0,00000000004
Write '[Purple] {DEC:dec}' to log
Will write "0,00000000006" to the log.
Obviously this isn't exactly a neat solution, but it could serve as a workaround for the time being.
EDIT: It does not appear this workaround is still available after the interface changes in v1.8