Add another property on contenttype that reads from first and returns a string array...?

I think Daniel's suggestion is a viable option, or you could customize the indexing if you don't want to modify your original object: http://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-Find/9/DotNET-Client-API/Indexing/

It's amazing how you sometimes don't see the the simple solutions for problems. Thanks for the input! Another property will solve this nicely.

Isn't it...? :)

Extension method for the content type is a slightly more elegant way of doing the same if you don't want to mess with your model btw. 

Yes that would probably be a better, I just have to figure out how to set that up with Vulcan.

Solved it for now using the following setup (adding a property on the page object):

[Display(
            GroupName = GroupNames.TechnicalInformation,
            Order = 700)]
        [CategorySelectMany("HouseProperties", SelectionFactoryType = typeof(CategorySelectionFactory))]
        public virtual string HousePropertiesString { get; set; }

        public IEnumerable<string> HouseProperties
        {
            get
            {
                var properties = this.GetPropertyValue(p => p.HousePropertiesString);
                if (string.IsNullOrEmpty(properties)) return new List<string>();
                return properties.Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
            }
        }

Tobias,

You can also add custom fields by implementing an IVulcanIndexingModifier. I've done that to add isSearchable to a page type in a nuget assembly. Here is a sample:

        public void ProcessContent(IContent content, Stream writableStream)
        {
            if (content == null)
                return;

            string searchDescription;
            GetDescription(content, out searchDescription);

            // fallback to teaser
            if (string.IsNullOrWhiteSpace(searchDescription) && content is SitePageData)
                searchDescription = ((SitePageData)content).Teaser;

            var streamWriter = new StreamWriter(writableStream);

            if (!string.IsNullOrWhiteSpace(searchDescription))
                streamWriter.Write(",\"" + VulcanFieldConstants.SearchDescriptionField + "\": " + Escape(searchDescription));

            // Add IsSearchable 
            if (content is BlogPostData)
                streamWriter.Write(",\"isSearchable\": true");

            streamWriter.Flush();

        }