You can both pick up and return a Pocket WiFi at Itami Airport, and it is very convenient.
Even after arriving at Itami Airport, you can rent a pocket WiFi.

However, we strongly recommend that you pre-order the pocket WiFi beforehand.
Let us quote a few words explaining a reason why you should pre-order pocket WiFi router.

If you are considering renting a pocket WiFi or buying a SIM card at stores at Itami Airport
unfortunately, you can not always rent/buy it because their stock may run out.
Especially during peak season (March, April, December), you highly likely to feel very uncomfortable because
you may find that there are no stock of pocket WiFi/Prepaid SIM or you have to wait in a long line for a long time.
Also, their prices are very high.

Itami Airport Pocket WiFi/Data SIM Rental Service

This is a quotation from the page of Itami Airport written at LightPocket,
one of Japanese companies offering pocket WiFi rental service.

VisitJapanPlaces has the same opinion on that.

In case you do not pre-order a Pocket WiFi and receive it at Itami Airport, you may have the following disadvantages:

That's why, we recommend that you pre-order a pocket WiFi rental.
Just by pre-ordering it online, you can smoothly receive a pocket WiFi at a reasonable price.
We've listed providers below, offering such rental service.

To receive a pocket WiFi through the providers below, there are two major places:
Airport Post Office and Dedicated Counter.

Major differences between them are:

Easiest Way to Return A Pocket WiFi: Just Drop It in Postbox
There are various methods for returning a pocket WiFi, and they vary depending on a provider.

However, some providers offer the easiest way to return among them.
They offer a prepaid return envelope,
and all you have to do is place all rental items into the return envelope, seal it, and then drop it in postbox anywhere in Japan.
Postboxes are located really at many locations such as airports, convenience stores (Lawson), and around stations.

Pocket WiFi Rental Services: Pickup at Itami Airport Post Office

Itami Airport Post Office
Business Hour : 09:00-17:00 (All year round)
Address :560-0036 Toyonaka osakakokusaikuko bunshitsu kyokudome,3-555 Hotarugaikenishimachi, Toyonaka-shi,
Itami Airport

LightPocket CDJapan Rental Global Advanced Commnication Japan Wireless Wi-Fi RENTAL Store Japan Mobile Rental Rental Wifi Japan-wifi Japan WIFI Rental ANYFi Global Mobile Easygo Japan GENKI MOBILE
Pocket WiFi Rental Services: Pickup at Own Service Counter / Etc. at Itami Airport
Ninja WiFi PuPuRu WiFi JAPAN WIFI Japan Rail Pass

Other Major Airports in Japan

Narita Airport Terminal 1 Narita Airport Terminal 2 Kansai Airport Haneda Airport Chubu Centrair Airport New Chitose Airport