As far as I could tell, there are no screen exits available in VL02N/VL03N to add buttons to the application toolbar.
However, there may be an acceptable workaround - but it is in no way as elegant as a button in the app. toolbar:
There are BAdIs to add tab pages in VL02N/VL03N on the header or item level. You could add a tab with a single button in it opening the required URL.
LE_SHP_TAB_CUST_OVER - Tab Page for Own Data - Item Detail Delivery
LE_SHP_TAB_CUST_ITEM - Tab Page for Own Data - Item Detail Delivery
As I said, not as pretty as a button in the app. toolbar, but it might work for you.