TestAction duration field supports only numeric values. Seems it is impossible to specify dynamic pause using Test Action. It is possible to put a Timer to TestAction element, but I think it will be great to allow variables/parameters in this field.
Yes, that's because the field is converted to an integer before it is stored in the test plan. But I agree, this is an unnecessary restriction. The field should be made bigger to allow for function calls. And the label should make clear that the value is in milliseconds.
Applied to SVN in r590093
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/2041