Is there a way to specify a wire inside FPGA?

Provided you don't try to connect your signal in a higher level module, you can drive an inout signal down to a lower module.

However normal practice is to do I/O driving in the top level, driving data and output enable signals as outputs to the top level and sending signals down to the inner levels.

example:

inout pad_sda;

inout pad_scl;

wire sda_oe_n;

wire scl_oe_n;

assign pad_sda = sda_oe_n ? 1'b0 : 1'bz;

assign pad_scl = scl_oe_n ? 1'b0 : 1'bz;

i2c_module i2c_inst (

.sda( pad_sda ),

.sda_oe_n( sda_oe_n ),

.scl_oe_n(scl_oe_n )

);

then in

module i2c_module (

sda,

sda_oe_n,

scl_oe_n

);

input sda;

output sda_oe_n;

output scl_oe_n;

....

endmodule