Eu preciso de um adaptador que se conecte à WLAN existente e ofereça um conector Ethernet para a impressora.
Use uma ponte sem fio:
A wireless bridge adapter, like the Asus WL330GE, simply "catches" the Wi-Fi signal and converts it to Ethernet, letting connect those Ethernet-only devices without a long cable run.
Just note that these products often involve some elaborate setup routines (such as installing software on a nearby PC, or accessing a setup screen through a web browser) that skew towards the skillset of an advanced user.
Fonte Como obter seus gadgets somente Ethernet em sua rede doméstica - CNET
Esse dispositivo em particular custa US $ 189,95 e é provavelmente um exagero para suas necessidades, mas há dispositivos mais baratos / mais simples disponíveis:
Laptone N300 Wireless Mini Router
Laptone Wireless Mini Router is designed to conveniently extend the coverage and improve the signal strength of an existing wireless network. With 300Mbps wireless N speeds, Range Extender button, miniature size and wall-mounted design, extending a wireless network has never been easier.
What's more, its Ethernet port allows the device to act as a wireless adapter to turn a wired device into a wireless one.
Preço típico £ 14,99.