The most accurate way to handle this is enter the emploee type twice, once for each type of work with an estimated hours for each type of work.
For instance, if I paid my maintenance foreman $16.50/hr for maintenance work, but $22.50/hr when plowing snow, I'd enter in them in field labor budget for something like:
- Maintenance Foreman - 1500 hrs - $16.50/hr
- Snowplow Drivers - 200 hrs - $22.50/hr
.. assuming they're going to work a lot less hours in snow. See the screenshot below for an example.