Hello @Kooii 

1. If you need an extender to build a Mesh network with an existing Deco system, please consider more Deco units as extenders, as Decos could build an integrated Mesh system: 

Here are some related FAQs that may help:
How many Deco units can I add to my Deco mesh network?
What are the differences between different models of Deco?

For more Deco inquires, please start a thread here on Deco sub forum .

2.  OneMesh Powerline device / Range extender belong to TP-Link OneMesh  ,which uses a different Mesh technology from Deco Mesh

When OneMesh Powerline device / Range extender work with Deco unit, it could work only as a common extender, but not build a mesh network with Deco. Please refer to Q1 for details: 
Several questions about the OneMesh™ and Mesh

https://www.tp-link.com/onemesh/product-list/