Combination mekko (variable width chart) with line charts

Short but useless answer -- yes, this should be possible in Excel.

We will need more detail from you before we can provide more specific suggestions. Tutorials that I found
https://www.contextures.com/excelmarimekkochart.html
https://peltiertech.com/variable-width-column-charts/
suggest that Marimekko charts are area/stacked area charts in Excel. It should be fairly straightforward to follow that up by adding additional series and converting those to line charts to make an area/stacked area + line combination chart. If it helps, note that Peltier's tutorial uses a line chart data series for the column/area labels. That should help show how to make the combination chart.

Are you already familiar with creating Marimekko charts in Excel? Are you familiar with making combination charts in Excel?

I'm not sure I understand your concern. Your "picture" looks to me like a basic area + line combination chart. I see no reason why the line chart series would be unable to cross over the area chart series. Without more detail, my best recommendation is to follow the tutorials for creating the Mekko charts, then add the data for the line charts and make the area + line combination chart.

As I asked before, are you familiar with creating combination charts in Excel?

Not knowing what you are stuck on, here is a detailed example of what I would expect to do:

0) Start with the sample file from the contextures tutorial (download sample file from contextures link -> unzip -> open spreadsheet -> navigate to "MarimekkoBasic" tab).
1) In R1 enter a suitable label ("line1"), In S1, enter a second label ("line2")
2) In R3:R5 (leave R2 blank), enter the desired value for the first step for line1. In S3:S5, enter the desired value for the first step of line2 (note how these cells correspond to the first "column" of the Mekko/area chart).
3) In R6:R9, enter the desired value for the second step for line1. In S6:S9, enter the desired value for the second step of the line2 (corresponding to the second "column" of the Mekko/area chart).
4) Continue entering the values for each step in columns R and S.
5) Select the chart -> Select data dialog -> add columns R and S as two new data series.
5a) While you have the Select data dialog open, click on the "Hidden and empty cells" button to bring up that dialog and make sure that the chart is set to use "gaps" as the zero and empty cells setting.
6) Select the "line1" data series -> change chart type -> line -> desired line subtype. Repeat for the "line2" series.

That should be really close to what you want. When you want to "remove part of the lines", you should just need to leave the appropriate segment in R or S empty.

Does that help you see how to add the line series to the mekko/area chart?

That suggests to me that you have something other than one of the "line" data series selected when you change the chart type. Be sure that you have only selected one of the line series before changing chart type to line. If there is any doubt, select one of the line data series from the drop down list of chart elements in the Chart tools -> Format ribbon.