I am a big fan of the CRM app.  It meets almost all my needs.  There are a few things missing for me which is why I am trying to create my own database.  My question is, is it possible to relate a custom database to Zoho CRM?  I can create a database to manage client and contractors and the services that bring them together, but I don't want to reinvent the invoicing and estimates components if I don't have to. 

What do I need to do so I can send an estimate or an invoice to a customer in my custom database? 

Any assistance or links will be greatly appreciated. 

DJ