I think it's not useful to mix price files, there are two solutions using prices from several brokers:

1. temporarily

You rename e.g. the original EURUSD_2018.t6 to EURUSD_FXCM_2018.t6 and download the prices from OANDA to an empty price file EURUSD_2018.t6. If you want to use FXCM prices you make the procedure vice versa. Alternatively you keep the price files with FXCM and OANDA and make a sym link (mklink on Win) from one to the EURUSD_2018.t6 depending on which broker you want.

2. permanently

You create an account.csv like:

Name,Broker,Account,User,Pass,Assets,CCY,Real,NFA,Plugin
Broker1, ..., NewAssets.csv, ...
Broker2, ..., NewAssets.csv, ...

and use asset names in the NewAssets.csv like

Name,Price,Spread,RollLong,RollShort,PIP,PIPCost,MarginCost,Leverage,LotAmount,Commission,Symbol
EURUSD_Broker1,1.09546,0.00005,-0.14,0.06,0.0001,0.09129,10,0,1000,0.6,Broker1:EUR/USD
EURUSD_Broker2,1.09546,0.00005,-0.14,0.06,0.0001,0.09129,10,0,1000,0.6,Broker2:EUR/USD

In this case the price files are already named correctly e.g. EURUSD_Broker1_2018.t6 when downloading history.