Adding an error to a ValidationSummary control via code-behind

I have a number of Validation Controls tied to various fields on a web form. I also have a ValidationSummary control which correctly displays all the errors on a postback. I also have a number of buttons on the form.

Here's my problem: when one of the buttons on the form is pushed I need to perform an additional (cross field) validation and then post an additional error to the ValidationSummary control if necessary.

Can errors be added to the ValidationSummary control directly in code-behind code or can they only be added via the use of Validation controls?

I'm trying to do this in code-behind because i want this particular validation to only fire if one of the (many) buttons is pressed.

Hope this makes sense!

Thanks. ...