Is it possible to connect hard wired devices to a portable generator avoiding a transfer switch or some other device connected to the main circuit breaker box?thanks in advance

No, it is not possible.

If you are planning on using a generator, you must use a disconnect switch that totally disconnects the service panelboard and power company service lines.

If you do not do this, your generator will backfeed the power company lines and as transformers are hit, the voltage and amperage being applied by your generator will be raised to lethal levels and has the potential of killing power company technicians who may be working on what they believe to be dead circuits.