How can an AC source be connected in parallel with a DC one (through a capacitor) in a common-collector BJT configuration?
an AC voltage source (Vin) is connected in parallel with a DC voltage
source (Vcc)

Not true - the AC source is capacitively coupled to a node of two resistors forming a resistor divider with Vcc and this is wholly different to connecting a pure voltage source to another pure voltage source.

If Vin would be greater than Vcc, could Vin be charging-discharging
Vcc?

There are bigger problems because if Vin were greater than Vcc it would stop the circuit working and may even damage the transistor.
Generally, when parallel connecting ideal voltage sources (whether AC or DC), you have to avoid infinities and that usually means using interconnection resistors.