Communicate Through Siemens Backplane

We've got several Siemens S7-400 processors that are connected to our plant network via ethernet. These processors also communicating to another S7-300 through the Profibus connection on the front of the S7-400. Is there a way I can get online with the S7-300's by going over ethernet to the S7-400 then through the processor to the Profibus network like you can with the Rockwell devices? I've played around with it for a little while but can't figure it out.

Yes there is a way but you have to configure the so called S7-routing

https://support.industry.siemens.com/cs/document/109474569/how-do-you-enable-cross-project-s7-routing-in-the-tia-portal-and-in-step-7-v5-x-?dti=0&lc=en-WW

https://support.industry.siemens.com/cs/document/584459/which-modules-support-the-s7-routing-function-in-s7-subnets-?dti=0&lc=en-WW

I will put it shortly:
All devices must be in the same STEP7 project.
In the STEP7 project there must be either a PC Station or a PG/PC Station (a simplified PC Station only used for programming).
The hardware configuration for this project must be compiled and downloaded to all devices (actually, it is only the devices in the middle that need to be downloaded, the ones that will do the routing).
When you have come this far, In NetPro you shall see something like this:

Notice the yellow arrow above the PG/PC Station. This is how NetPro tells you that it identifies the PC as "itself".
You should then be able to go online with any device on the network.