Edit pre-built widget from theme..?

Hey there,

I’m building a theme for a huge, very established website that is moving to WordPress in a couple months.
Problem is, I’m relatively new to WordPress themeing. It’s going great so far, but I’ve hit a wall.

I’m using default widgets in my sidebar with the_widget().
Problem is, the_widget() doesn’t show the widgets in the back-end (Appearance -> Widgets). I want something similar, so that my client can edit my ‘prebuilt’ widget that comes with the theme.

Edit: It would be a sort of ‘install widget on theme activation’, I suppose.

Any ideas?
A huge thanks for your time.

Harley.