You may want to start here (Mission Planner Dev) and then follow the links to the developer related community(s). I'm sure if there is a work-around to your question, they'll have the answers.
On the MP Flight plan screen at the bottom where it shows the Waypoints the first line has some defaults and check boxes. The last check box to the right should be labeled "Spline"; check the box and all the WPs added fromm that point will be splined unless you uncheck the box or select the first drop down for a regular WP.
If you don't have that box then you need to update or reinstall MP.
I could assume the data file could be edited, Save the mission and then open it as a txt file, a standard WP is 16 and a splined WP is 82 in the example file I just created... I changed all 4 WP to 82 and re-opened in MP, the WP updated to Splined WP....