Yes, it is entirely possible to combine pre-event (pre-match) selections and live betting (in-play) selections in your bet slip.
This combination is permitted for both Multiple Bets (Accumulators) and System Bets (E.g., Yankee, Trixie).
How to Combine
Choose Selections: Add the Odds from the pre-event markets and the Odds from the live markets you wish to combine to your bet slip.
Specify the Type: Choose the type of bet (Multiple, Trixie, etc.).
Confirm: Click the "BET NOW" button (or equivalent) to finalize the wager.