There are two approaches that I know of:
Define a calculation similar to:
If(additional training = "yes"; ThefieldWhoseValueYouWantInthelist; "")
Now define your value list ot refer to this field for your value list.
Define a self-join relationship that links a field that always contains "yes" to your additional training field. Define your value list to specify your value field in it's original table, select the "only include related values from" option and choose the other table occurrence of your self-join relationship.