how to add a methyl group to a straight carbon chain
Why would you want an expensive process to convert cheap molecules into other cheap molecules?  Brominate ethane, make the organo-lithium, displace methyl iodide. Epoxidize ethylene, react with methyl lithium, eliminate to the olefin. Silly.

Doing something cheap, easy, and fast on a huge scale with unwanted methane from oil wells to form a high energy density liquid for pipeline conveyance will make you rich. Methanol is marginal - and still can't be done.  Perhaps methane plus air into titanium-silicalite (plus hydrogen peroxide, or cobalt- or vanadium-doped plus air) then that mixed product through ZSM-5 into aromatic gasoline.  That could be exciting in various ways.

http://dx.doi.org/10.1016/j.apcata.2008.04.020 
http://chemelab.ucsd.edu/methanol/memos/final.html