Make sure your [Date imported mod.] field is of type Date. In Tools > Configure Auto-Import..., rules, create an import rule that assigns [Date Imported] to [Date imported mod.].
Now, when the files are imported, [Date imported mod.] will have the imported date, which you are free to change later.
In your views or playlists, use [Date imported mod.] instead of [Date Imported].
To bootstrap yourself, you can quickly assign [Date Imported] to [Date imported mod.], by selecting all the files, editing the [Date Imported mod.] field (e.g. in the Tag Action Window, or in place in the pane via F2), and entering:
=[Date Imported]
Try it on a few first.