Overriding Core files

Hi, i am developing a site in boonex dolphin. Due to some requirement, there is some needs to enhance the functionality in core files like,

1. member.php

2. join.php

3. inc/design.inc.php

4. inc/classes/bxdoltemplate.php

5. inc/classes/bxdolprofilefields.php

6. inc/classes/bxdolpfm.php etc...

But as we know, that changes in core files is not a good idea, so how to override these files to add my functionality. Also if I want to modify some core modules, how to override that also.

Thanks n Regards

Nisha Sharma

though, i could be incorrect on this, but I would think you would need to create modules, that would extend these functionalities thus leaving core intact, but adding your additional required features and functions. 

but yea, editing core files is not the best method, when considering the future, since all edits on the core will be overwritten on updates.