Of course it is! Don't be afraid to reduce gain down to the lowest possible position. The preamps are able to handle all input levels. Actually the RML mixers don't really need line level inputs. They are only meant to replace up to 8 separate DI boxes, which is a nice feature.
One thing to check is the output impedance of your line outputs. You want it to be significantly lower than the 1 kOhms mic input impedance, say 200 Ohms or less. Higher values may lead to signal distortion. In that case a DI box is the way to go.