Hi, welcome to GoY, sow the seed as soon as ripe, or in late winter or early spring, using seed trays or pots, and leave them outside in a sheltered position, or divide large plants between autumn and early spring, you can also take basal cuttings in autumn or early spring, or root cuttings in winter, while the plant is dormant.
I don't think you can extend the flowering season, they flower in summer and that's it, Derek.

Agree with Derek you can't extend the flowering season with any primula.