I am not sure to follow you.
SixSigma is a is a methodology to manage process variations that uses data and statistical analysis to measure and improve a company's operational performance.
So take any process (SDP or other), choose what you want to measure, identify the issues, plan the solutions, evaluate the impacts.
The SixSigma projects I have participated were all fairly transversal and not linked to a software life cycle.
By transversal, I mean transversal to the "product design-development-construction-delivery" process that is software development.
For example, in an environment were we need to produce a set of programs running on our internal production platform, most of our SixSigma projects are centered around Operational Architecture, that is "making operational an execution environment" (how to set up servers and networks in order to stop, update, install and launch a set of executables, and that for many projects each with their own SDP).
That is a notion transversal to any SDP you want, since in the end, all those "Development Processes" have but one goal in common: put your software into production.
The criteria to measure were precise and reproducible, going from the time to manage the merges needed to consolidate a final executable to the number of merge errors to the deployment errors (because of an incorrect labels or a faulty release notes).
All those missteps were noted release after release, and the goal was to reduce them.
One side-effect was to identify an inadequate merge workflow, workflow which, once fixed, allowed us to greatly reduce the errors in the final set of deliveries.