A command to append a value to a field, rather than replace all values

Explain the problem as you see it

I'm creating a command right now to set a node's "Pertains to" field to a specific project that I'm focusing on, and that field will often have other values. My command replaces those values, rather than appending them.

Why is this a problem for you?

Because the other values already in the field are important and there for a reason.

Suggest a solution

Either:

1. "Append value" command
2. Additional configuration option in the "Set field values" command that allows me to select whether I'd like to replace the value or append to the list of values.