Yes, you can absolutely add an Universal Express Pass to your trip even after you have already purchased your standard admission tickets. Universal Orlando and Universal Studios Hollywood offer several ways to do this: you can buy them through the official mobile app, on the website by logging into your account, or in person at the front gate or various kiosks located throughout the parks. It is important to know that Express Pass pricing is "date-based" and "dynamic," meaning the price increases as the park gets more crowded and as the remaining inventory decreases. Therefore, if you decide to wait until you are inside the park to see how long the lines are, you might end up paying a much higher price than if you had added it a few days prior. Additionally, if you are staying at a Universal Premier Hotel (like Portofino Bay or Hard Rock), you don't need to "add" or "buy" the pass at all, as the Unlimited Express Pass is included for free as a benefit of your stay. For everyone else, adding the pass later is a standard procedure, but doing it earlier is almost always more cost-effective.