This is just what I wanted.
An example would be
import torchvision
import torch
import specialconv

frcnn = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True)
prev_weight = frcnn.backbone.body.conv1.weight
prev_bias = frcnn.backbone.body.conv1.bias
new_layer = SpecialConv2d(...) # make sure args match
new_layer.weight = prev_weight
new_bias = prev_bias
frcnn.backbone.body.conv1 = new_layer