The Beijer HMI panels I have worked with use ModbusTCP over ethernet for communication. As long as the ethernet protocols are the same between the Siemens PLC and the HMI (Profinet/Profinet, EthernetIP/EthernetIP, or ModbusTCP/ModbusTCP), you should be ok.
In order to use Simatic Manager or TIA Portal to manage tags from connected devices, they need to be a part of the hardware configuration for your project. As long as the proper hardware is selected to make a connection to your devices, you should be able to configure a communication connection and access the tags. In your case, you will need to add the PLC that is in a different project to your HMI project. Assuming you wish to maintain the PLC program in the separate project, just create the hardware configuration for the PLC in the HMI project, export the tags from the PLC project and then import the tag table into the HMI project.