I assume you would like to add the new linear layer at the end of the model?
If so, resnet50 uses the .fc attribute to store the last linear layer:
model = models.resnet50()
print(model.fc)
> Linear(in_features=2048, out_features=1000, bias=True)

You could store this layer and add a new nn.Sequential container as the .fc attribute via:
lin = model.fc
new_lin = nn.Sequential(
    nn.Linear(lin.in_features, lin.in_features),
    nn.ReLU(),
    lin
)

model.fc = new_lin