Underground feeder short by 5ft, Main panel to subpanel

You can make a splice in an appropriately sized junction box indoors. Given the large size of the wire, there will be certain minimum box size specs. The box will be long and thin if you do nose-tail connections of the wires. The electrical supply house should be able to help you with box size.

The type of splice used is called a Polaris. Take care to torque it to spec. Select Polaris connectors just large enough for your wires, so the insulation fits the connector tightly.