The staffing boost comes as Bulgaria prepares to partially join Europe's open-travel Schengen Zone, a move which will scrap passport checks for people travelling between the country and that area by air and sea from March 31.
Border checks for land travel will remain.
The staffing boost would bring the number of officers working there to between 500-600, Frontex chief Hans Leijtens said, standing near the frontier's barbed wire-topped fence.
"Bulgaria is a very important partner. Without Bulgaria we cannot protect our external borders," he added.
Frontex said it recorded 380,000 irregular crossings across all borders into the European Union last year, the highest since 2016.
Overall numbers are still way down from a peak in 2015 and 2016 which was fuelled by a rise in migrants fleeing war and poverty in the Middle East and Africa.
(Reporting by Stoyan Nenov; Writing by Ivana Sekularac; Editing by Andrew Heavens)
