Can a subpanel box be used as a junction box or splice box

No, a subpanel box cannot be used as a junction box.

The real answer is yes, a panelboard, (there is no technical term "subpanel" listed in the National Electric Code) can contain splices.

If there is adequate space, the wiring shall not take up more than 40% of the cross-sectional area of that space, and the splices shall not take up more than 75% of that space.

This is all listed in Section #312.8 of the National Electric Code.