Set choice field values in initiation form using workflow variables SPD2010?

If what you really need is adding a dropdown menu with values coming from a given query/data source, then I think the easier thing you can do is editing the initiation form in InfoPath.

I would try this:

1. in SPD, declare a new initiation form parameter of type single line of text or number, depending of the data type you need to get back once a choice has been made
2. Save and publish. You should see a .xsn file listed under Forms. Click on it and InfoPath will open.
3. Right click on textbox field you should see on the form and change the control type to Drop Down List Box.
4. Right click on the dropdown, then click on DropDown List Box Properties. Under List box choices, select Get choices from an external data source, then Add.
5. Configure the new datasource to retrieve items from a SharePoint list and select the list where the values you want to show are stored.
6. Select the fields to include in the query results (eg. Title and ID), specify an optional sort order, then Finish.
7. Back in the DropDown List Box properties panel, set Value and Display name fields.
8. Save and publish.

Another solution is tampering with the XOML file generated by SPD, which is obviously not officially supported. You can find more info on this blog post.