SYSUTCTRANSBEGINTIME

You can try converting the sys.dm_tran_active_transactions.transaction_begin_time column to UTC but you will NOT get the datetime2 output.

SELECT
    transaction_id,
    transaction_begin_time AS local_time,
    DATEADD(
        MINUTE,
        DATEDIFF(MINUTE, SYSDATETIME(), SYSUTCDATETIME()),
        transaction_begin_time
    ) AS transaction_begin_time_utc
FROM sys.dm_tran_active_transactions;