Insulation

Yes.

bhalchandra

As already stated you can insulate an FRP tank.

But, You must remember that insulation by itself will not keep your fluid at the desired temperature unless there is some form of heat input. This can be by circulation to an outside heater or by an in-tank heater.

The insulated tank with no heat input will, over time seek equilibrium with the ambient temperature surrounding the tank.

To add to the fine advice given above....

Yes, it is possible to add insulation, but this requirement should be considered at the time of tank purchase.

Metal and FRP tanks can be equipped with stiffeners and tie-points that will greatly ease the installation and attachment of insulation.

Was this tank used for some other service in the past ? (silly question...)

I'd have it sprayed with two part urethane foam. Then if a more durable finish is needed, you can paint the foam.

You may still need a heater.