To convert to/from wrapped fungible tokens, for example, ETH to WETH, use sdk.balances.convert function:
import { Blockchain } from "@rarible/api-client"
//Convert 0.1 ETH to 0.1 wETH (Wrapped Ether)
const tx = await sdk.balances.convert(Blockchain.ETHEREUM, true, "0.1")
await tx.wait()
//Or unwrap 0.1 wETH to 0.1 ETH
const tx = await sdk.balances.convert(Blockchain.ETHEREUM, false, "0.1")
await tx.wait()