How can a User add their own coding with my function??

Question

Hello Everyone,
I have already created a user control with few functionality. I want my user can able to write their own coding with addition to my coding.
for example...Internal implementation of Page_Load() will be some coding ,and if i write any thing in Page_Load..then the Page_Load executes both the internal as well as my coding...How can i make such kind of function.
How can i make this possible??
 
Thanks in advance.