Please help us help you and fill where relevant:
Your LimeSurvey version: 6.5.4
Own server or LimeSurvey hosting: aeonfree
Survey theme/template: Fruity
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I want to add a new separate function in the Limesurvey.
This function will be implemented in one separate php file that I 'd like to link to other Limesurvey php pages.
That new function will add/edit and display some data from the Limesurvey database.

Do you think this is possible?

Thank you in advance 

Check this
www.limesurvey.org/manual/Add_new_menu_and_view_by_a_plugin

And some sample
gitlab.com/SondagesPro/coreAndTools/backupSurveyLocally
( gitlab.com/SondagesPro/coreAndTools/back...?ref_type=heads#L153 )