Hello Thomas,
Add-on customization is of course possible upon the standard C4C logic through PDI[Partner Development Infrastructure]. The developers can use the Cloud Application Studio to build these add-on solutions.
You may follow the Cloud Application Studio SCN space @ http://scn.sap.com/community/business-bydesign/studio to keep yourself updated and pose questions.
The space is common for SAP Cloud Solution built on the ABAP Netweaver stack as Cloud Application Studio is the common SDK used on for these applications.\[ SAP Business ByDesign, SAP Cloud for Customer, SAP Cloud for Travel and Expense, and SAP Cloud for Financials].
Refer this help documentation: SAP Cloud Applications Studio – SAP Help Portal Page
I hope this helps.
Thank,
Suraj