Minecraft wiki entry for World_border, associated text: Add
/worldborder add <sizeInBlocks> [timeInSeconds]
Adds or subtracts sizeInBlocks to/from the current world border width and length. sizeInBlocks may be a positive or negative number. Optionally, a timeInSeconds may be specified such that the border grows or shrinks from the current width to that being set over the specified time in seconds.