Customize Global Quick Create

Hello,

Is there a way to modify the Global Quick Create menu. If you enable the Quick Create Form for an entity, it appears in Quick Create menu i.e. + sign menu. But there are some entities which does not allow you to activate quick create form.

I wanted to add Fax and Letter Activity in Quick Create menu but for these Allow Quick Create property is grayed out.

Is there a way we can add it.

Its cannot be customized through Sitemap or Application either.

Thanks

Milind