Yes, I am retired from Rohm and Haas. I am working part time for Chilworth Technologies as a consultant, and also with AIChE/CCPS.

With regard to the question, I am hesitant to recommend a specific procedure because I have never had any experience with sulfur trioxide, so my knowledge of the hazards is limited to what I can read on line. There is enough there to concern me, though.

From an ILO safety sheet at http://www.ilo.org/public/english/protection/safework
/cis/products/icsc/dtasht/_icsc12/icsc1202.htm
:

"When the alpha form melts it takes the gamma form, and vapor pressure rises dramatically with a hazard of explosion. Melting point is 62, 33 and 17°C for alpha, beta and gamma forms. Vapour pressure is 9.7, 45.9 and 57.7 kPa at 25°C for alpha, beta and gamma forms."

Some other sources for what to do if you receive lab quantities of sulfur trioxide with solid present is to heat slowly over a long period of time, with the temperature never to exceed 35 C, and that it could take a week to melt the material. For example, the Sigma-Aldrich technical bulletin for lab material (attached) says:

"If material is not completely liquid ( it may contain low melting solid polymer), place bottle in a glass beaker or metal pail and warm under heat lamp or in oven at 35°C (95°F). DO NOT OVERHEAT!! Sulfur trioxide, stabilized, boils at 44.7°C 112.6°F). Melting may take up to one week. When the material is liquid, it is ready for use. Traces of non-melting polymer should not interfere with the performance of the liquid material."

But, this is for lab quantities. You would need to think about how to translate this procedure to a specific production scale storage tank.

When faced with a question like this, my advice is always to immediately contact the supplier, or other suppliers or manufacturers of the material. They have a much more extensive experience with dealing with issues such as this, both in their own facilities, and working with other customers. They may have a standard written procedure for dealing with this kind of an issue if it comes up often enough. And they certainly have access to experts with a lot of experience with handling the material for many years. For example, Rohm and Haas and other manufacturers of acrylic acid have very specific procedures for safely melting frozen material. I have found that manufacturers are usually very cooperative in helping avoid incidents.