This ended up being easier than expected. Here is the code to do this:
public static ICreateTableColumnOptionOrWithColumnSyntax WithUser(this ICreateTableWithColumnSyntax tableWithColumnSyntax)
{
return tableWithColumnSyntax
.WithColumn("UserId")
.AsInt32()
.Nullable()
.ForeignKey("Users", "Id");
}
I just wish this was better documented somewhere.