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.
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