You can only add static abilities that change the characteristics of permanents, and a couple of "primitive" abilities.
A primitive ability is only a single line: "flying", "first strike" or "haste".
Other abilities consist of one or more lines that define the "subjects" and one or more lines that define the "changes" to those subject's characteristics.
Defining subjects. A subject may be this (meaning 'this permanent') or a set of permanents that satisfy all requirements on a line. These requirements may be a type (such as 'creature'), a subtype (such as 'Bear'), a color (such as 'red'), a lack of these characteristics (such as 'nonblack' or 'noncreature') or the word 'youcontrol' or 'nocontrol'.
Defining changes. A line such as "1/2" means adding "+1/+2" to the subjects; otherwise changes may be the following directives:
- switch switches power and toughness
- silence: lose all abilities
- losecolors: lose all colors
- setcolor:[colorname]: becomes [colorname]
- addcolor:[colorname]: is [colorname] in addition to other colors
- addtype:[cardtype]:is [cardtype] in addition to other types
- setpt:X/Y: sets base power and toughness
- gainscontrol:2: opponent gains control
- addability:flying: has flying
Other features may be undocumented.