Creating synchronized USRP source block

Hello,
I am trying to create a synchronized usrp source block in gnu radio consisting of multiple B210 USRP devices. Lang: C++.

From what I have found I need to:

• Instantiate multiple multi_usrp_sptr as each B210 requires one and multiple B210 devices cannot be addressed by using single sptr
• Use external frequency and PPS sources - an option that can be selected from block or set programmatically
• Synchronize re/tuning to achieve repeatable phase offset between nodes - this can be achieved using timed commands API https://kb.ettus.com/Synchronizing_USRP_Events_Using_Timed_Commands_in_UHD
• Synchronize sample streams using time_spec property issue_stream cmd

The problem is how should I insert these timed commands and set time_spec of stream in GNU radio block or gr-uhd libs?

I looked into the gr-uhd folder where the sink/source code resided and found functions that could be altered.
Unfortunately I don't know how to copy or export this library to do these modifications and later compile to  insert my custom blocks to GNU Radio, because gr-uhd seems to be built in and compiled at GR installation.

I attempted coping and then making the lib but that's not the way - it didn't succeed. Should I add my own source block via gr_modtool and insert only the commands I need?

Compatibility with uhd and its functions apart from just adding a few lines would be advantageous not to write the source from scratch.

Please advise,

Kind Regards,

Marcin Wachowiak