There's a few ways to go on the line side. In increasing order of cost/difficulty:
1) Inside the existing switchgear.
2)Add a tap box ahead of the main.
3) Add a new main switchgear ahead of the existing main. This really becomes a load side connection.
I've also brought a new PV service straight from the transformer secondary if the site was primary metered or a dedicated PV meter was preferred (i.e. Remote net metering)
Am I understanding that the panel will not accept a back feed breaker? What size panel- etc, it may be easier to change the panel, however, I get the feeling that is not a possibility.