I am not a Cakewalk employee.
During installation RPro checks the registry for Cakewalk Sound Center, DPro and Rapture installation information. If it finds those programs, the installer copies the DPro and Rapture programs into the RPro program folder (default path - "C:\Cakewalk Content\Rapture Pro\Programs") and updates registry entries in
HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\Rapture Pro\2.0. Note: this is a one time event. If CSC, DPro or Rapture are installed after RPro, they will not be added to RPro automatically.
RPro also includes the original DPro content in two installers under RPro in the Command Center and
your account called "D-Pro Part 1" and "D-Pro Part 2." The default path for the samples from these installers is "C:\Cakewalk Content\Cakewalk Sample Data\Rapture Pro\Multisamples"
The free DPro Expansion packs are available
here. I have not tried to install these without DPro installed.